title: Neuromuscular blockade of slow twitch muscle fibres elevates muscle oxygen uptake and energy turnover during submaximal exercise in humans.
abstract: We tested the hypothesis that a greater activation of fast-twitch (FT) fibres during dynamic exercise leads to a higher muscle oxygen uptake (VO2 ) and energy turnover as well as a slower muscle on-kinetics. Subjects performed one-legged knee-extensor exercise for 10 min at an intensity of 30 W without (CON) and with (CUR) arterial injections of the non-depolarizing neuromuscular blocking agent cisatracurium. In CUR, creatine phosphate (CP) was unaltered in slow twitch (ST) fibres and decreased (P < 0.05) by 28% in FT fibres, whereas in CON, CP decreased (P < 0.05) by 33% and 23% in ST and FT fibres, respectively. From 127 s of exercise, muscle VO2 was higher (P < 0.05) in CUR compared to CON (425 +/- 25 (+/- S.E.M.) versus 332 +/- 30 ml min(-1)) and remained higher (P < 0.05) throughout exercise. Using monoexponential fitting, the time constant of the exercise-induced muscle VO2 response was slower (P < 0.05) in CUR than in CON (55 +/- 6 versus 33 +/- 5 s). During CUR and CON, muscle homogenate CP was lowered (P < 0.05) by 32 and 35%, respectively, and also muscle lactate production was similar in CUR and CON (37.8 +/- 4.1 versus 35.2 +/- 6.2 mmol). Estimated total muscle ATP turnover was 19% higher (P < 0.05) in CUR than in CON (1196 +/- 90 versus 1011 +/- 59 mmol) and true mechanical efficiency was lower (P < 0.05) in CUR than in CON (26.2 +/- 2.0 versus 30.9 +/- 1.5%). In conclusion, the present findings provide evidence that FT fibres are less efficient than ST fibres in vivo at a contraction frequency of 1 Hz, and that the muscle VO2 kinetics is slowed by FT fibre activation.
authors.affiliation: Department of Exercise and Sport Sciences, Section of Human Physiology, The August Krogh Building, Universitetsparken 13, DK-2100 Copenhagen Ø, Denmark. pkrustrup@ifi.ku.dk
authors.forename: Peter
authors.initials: P
authors.lastname: Krustrup
authors.forename: Niels H
authors.initials: NH
authors.lastname: Secher
authors.forename: Mihai U
authors.initials: MU
authors.lastname: Relu
authors.forename: Ylva
authors.initials: Y
authors.lastname: Hellsten
authors.forename: Karin
authors.initials: K
authors.lastname: Söderlund
authors.forename: Jens
authors.initials: J
authors.lastname: Bangsbo
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000328:Adult; D001279:Atracurium / Q000008:administration & dosage / Q000031:analogs & derivatives / Q000494:pharmacology; D004734:Energy Metabolism / Q000187:drug effects*; D015444:Exercise / Q000502:physiology*; D006003:Glycogen / Q000378:metabolism; D006801:Humans; D007269:Injections, Intra-Arterial; D019344:Lactic Acid / Q000378:metabolism; D008297:Male; D018656:Muscle Fibers, Fast-Twitch / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D018657:Muscle Fibers, Slow-Twitch / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology; D003473:Neuromuscular Nondepolarizing Agents / Q000008:administration & dosage / Q000494:pharmacology*; D010100:Oxygen / Q000378:metabolism*; D010725:Phosphocreatine / Q000378:metabolism; D052097:Quadriceps Muscle / Q000098:blood supply / Q000187:drug effects / Q000502:physiology; D012039:Regional Blood Flow / Q000187:drug effects / Q000502:physiology; D055815:Young Adult
chemical_list: D003473:Neuromuscular Nondepolarizing Agents; D010725:Phosphocreatine; D001279:Atracurium; D019344:Lactic Acid; D000255:Adenosine Triphosphate; D006003:Glycogen; C101584:cisatracurium; D010100:Oxygen